The development of photovoltaic panels has significantly expanded in recent years with a view to the constantly increasing use of renewable energy sources in order to reduce the harmful greenhouse effect caused especially by carbon dioxide emissions.
This is also true of renewable energy sources such as wind or thermo-electrical sources.
These energy sources have the special feature wherein the electrical energy that they provide varies greatly according to the natural phenomena that supply them. Furthermore, a photovoltaic generator is a generator whose characteristic I=f(U) is highly non-linear. Thus, for a same illumination value, the power delivered will be different depending on the load.
Thus, the efficiency, i.e. the power delivered by a photovoltaic cell, depends not only on its exposure to sunlight which varies during the day, but also for example on the concealment of sunlight, for example by shade cast by clouds or other meteorological phenomena.
Now, when photovoltaic cells are for example connected to a load such as a consumer (for example a sensor or again a battery to be recharged), it turns out that the power transferred to the load does not generally correspond to the maximum power that could be delivered by the cell. Similar problems can be noted for wind energy. The result of this is not only that the efficiency drops for example because of weaker sunlight, but also that this efficiency is adversely affected by an imposed operating point situated below the potential performance of the cell.
In order to overcome this drawback and produce energy that is constantly as close as possible to the optimum operating point, circuits are used implementing a method known as the maximum power point tracking (MPPT) method which has been known since 1968. This is a method providing a better connection between a non-linear source and an arbitrary load.
These circuits are designed to force the generator, for example the photovoltaic cell, to work at its maximum power point, thus inducing improved efficiency.
An MPPT controller therefore drives the static converter connecting the load (a battery for example) and a photovoltaic panel so as to permanently provide maximum power to the load.
There are known ways, for maximum power point or MPP tracking, of applying a method based on a “perturb-and-observe” (P&O) approach.
In the case of a photovoltaic application, this is in fact an algorithm which, for a fixed voltage U1, will measure the corresponding power value P1 delivered by the generator, and then, after a certain period of time, dictate a voltage U2=U1+ΔU and also measure the corresponding power value P2. Thereafter, a voltage U3=U2+ΔU, is imposed if P2 is greater than P1. If not U3=U2−ΔU is imposed.
However, this implies current measurements and also substantial computation resources, for which the energy consumption is not negligible. This is why, in a large-sized photovoltaic installation, a sub-group of cells is dedicated exclusively to providing the energy needed for the control of the MPPT circuit.
However, in electronic micro-systems, such as for example autonomous sensors, this approach is not acceptable because the constraints in terms of space requirement and weight are great and it is necessary to have a system that is as small as possible with increased autonomy.
There also exist known maximum power point tracking circuits that possess an additional driver cell. This is not always desirable.
There also exist MPPT circuits without driver cells based on open-circuit voltage sampling performed by disconnecting, at fixed frequency, the photovoltaic panel from the rest of the circuit to measure the voltage in an open circuit. The system then reconnects the panel to the harvesting circuit which has taken the new optimized parameters into account. However, this results in frequent interruption of the energy harvesting process, which is unacceptable for electronic micro-systems that have to be autonomous.
Finally, there is a method known from the article “A simple single-sensor MPPT solution” in IEEE Transactions on Power Electronics, Vol. 22, No. 2, March 2007, describing an MPPT tracking method based on measurements of voltages.
This article explains that the derivative of the operating voltage of the energy source as a function of the duty cycle has a maximum value around the maximum power point MPP. The result of this is that tracking the maximum value of this voltage derivative is equivalent to tracking the maximum power point.
Thus, through simple measurements of voltages and operations of subtraction and comparison, it is possible to make the converter work around the maximum power point MPP.
To this end, the duty cycle α is made to vary at predefined intervals, for example at about 10 Hz, by a predefined quantity Δα, and the voltage derivative, i.e. the progress of the differences in voltages occurring after changes in the duty cycle, is tracked.
Now, in the above-mentioned article, this MPPT control loop is set up by means of a microcontroller.